a) Field of the Invention
The present invention relates to an objective lens system for use in endoscopes having small outside diameters such as bronchoscopes.
b) Description of the Prior Art
Endoscopes, bronchoscopes and the similar scopes must have especially small outside diameters. For this reason, an objective lens system to be used in such endoscopes must also have a small outside diameter and cannot contain an image guide fiber bundle or solid-state image pickup device having many picture elements, whereby no strict restrictions are imposed on the imaging performance of the objective lens system. Accordingly, it is desired to use, in the above-mentioned type of endoscopes, objective lens systems which are advantageous in manufacturing and assembly of the lens components thereof, and lens systems having simple compositions, as exemplified by the objective lens system for endoscopes disclosed by U.S. Pat. No. 4354734, are selected in practice.
The above-mentioned objective lens system for endoscopes comprises a first lens component consisting of a cover glass and a plano-convex lens element which are cemented to each other with a stop interposed, and a second lens component consisting of a plano-convex lens element arranged so as to locate the convex surface thereof on the object side. This objective lens system is advantageous that it permits the utilization of the entire diameter of the lens system as the effective diameter allowing light to pass therethrough without being restricted by a spacer. It is unnecessary to interpose a spacer between the lens system and an image guide fiber bundle or a solid-state image pickup device when the second lens component, which is designed as the plano-convex lens element having a small diameter but easily manufacturable, is cemented to an image guide fiber bundle or a solid-state image pickup device. The second lens component having such a composition has the roll of a field lens and functions to prevent the principal ray from being inclined on the image surface when the exit pupil is located sufficiently far, i.e., when the objective lens system is so designed as to be a nearly telecentric lens system. This design serves to prevent loss of light quantity from being increased due to oblique incidence of the light bundle on the marginal portion of the image guide fiber bundle in fiber scopes.
In recent years where wide angles are demanded for the endoscopes such as bronchoscopes, field angles larger than 80.degree. are mainly selected even in the field where endoscopes having thin diameters are applied. Accordingly, problems are posed by aberrations of pupil which are not problematic in the conventional objective lens systems having narrower field angles.
FIG. 1 illustrates a sectional view of the objective lens system, with the cover glass excepted, disclosed by U.S. Pat. No. 4354734, traced in which are passages of the rays emerging from the individual points on the image surface in parallel with the optical axis at an image height set so as to obtain a field angle of 120.degree..
This objective lens system has the numerical data listed below:
r.sub.2 =.infin.
d.sub.2 =1.7157 n.sub.2 =1.883 v.sub.2 =40.78 PA1 d.sub.3 =0.1667 PA1 d.sub.4 =1.8318 n.sub.3 =1.883 v.sub.3 =40.78 PA1 f=1
r.sub.3 =-1.7157
r.sub.4 =1.6621
r.sub.5 =.infin.
The numerical data listed above are different from those of the lens system disclosed by U.S. Pat. No. 4354734 and obtained by modifying the numerical data described as a preferred embodiment so as to obtain optimum aberrations within the range of the conditions described by U.S. Pat. No. 4354734.
The front focal point of this lens system is located at the position of the stop S shown in FIG. 1.
As is seen from the rays illustrated in FIG. 1, spherical aberration is at a degree posing little problem when image height is low and field angle is narrow. When image height is enhanced to widen field angle, however, the spherical aberration of the pupil abruptly increases thereby largely deviating the converging location of the rays having high image heights from the stop. Accordingly, the objective lens system is not completely telecentric for the rays having high image heights. This telecentricity is illustrated in FIG. 2, showing passages of the rays transmitted through the center of the stop arranged in the lens system having the numerical data listed above. As is clear from this drawing, the exit pupil for the paraxial ray is located at an infinite distance, whereas the exit pupil for the off axial ray for large image height is located in the vicinity of the image point under the influence of the the spherical aberration. For this reason, angle of emergence of the principal ray abruptly increases as image height enhances, thereby increasing loss of light quantity at the marginal portion of the image guide fiber bundle during transmission. Further, in an endoscope such as a non-flexible endoscope using a relay lens system, the pupil for an offaxial object point is not relayed to the side of the eyepiece lens and the off axial ray is eclipsed in the course of the relay lens system, thereby similarly increasing loss of light quantity at the marginal portion of image.
Furthermore, in case of an endoscope using a single color solid-state image pickup device, color shading is produced for the reason described above. Specifically, color filters are arranged on the surfaces of the individual picture elements of the single color solid-state image pickup device. These color filters are not in complete contact with photoelectric transducer elements of the image pick up device, and an airspace having a certain width remains between the filters and the transducer elements. Only the rays having passed through the filters should originally reach the photoelectric transducer elements but, when rays pass obliquely in practice, rays which have not passed through the filters reach the photoelectric transducer elements and degrade color reproducibility. Accordingly, the telecentricity is demanded not only for the objective lens system to be combined with an image guide fiber bundle but also for the objective lens system to be used with the single color solid-state image pickup device.
As an objective lens system having the composition similar to that of the objective lens system according to the present invention, there is known the Embodiment 7 of the lens system disclosed by Japanese Unexamined Published Patent Application No. 162021/61. This objective lens system is designed so as to correct distortion by using an aspherical surface and simultaneously correct the curvature of field produced by the aspherical surface with another aspherical surface. For this purpose, the objective lens system is designed so as to have functions for correcting distortion which are opposite to each other on both the sides of a stop. Specifically, the aspherical surface arranged before the stop has such a shape as to strengthen the positive refractive function (or weaken the negative refractive function) at the marginal portion of the lens, whereas the aspherical surface arranged after the stop has such a shape as to weaken the positive function (or strengthen the negative function). Accordingly, this objective lens system has a composition which is devided into a front lens unit and a rear lens unit with a stop interposed therebetween, and comprises at least one aspherical surface in each of the lens units.
The above-described objective lens systems for endoscopes comprising a stop in the vicinity of the front focal point thereof can hardly lower inclination of the principal ray emerging from the lens systems when the lens systems have large field angles, and have the above-described defects when used in endoscopes comprising image guides or solid-state image pickup devices.